A balance with electromagnetic force compensation which includes a controller device with a differential measurement circuit belongs to the know state of the art and is disclosed for example in [1], DE 101 53 603 A1. The balance described there and as illustrated hereinafter in FIG. 1 includes a cup-shaped permanent magnet system 109 with an air gap in which a coil is arranged. The latter is connected to a movable measurement lever 106 and conducts the flow of a compensating current Icomp whose magnitude depends on a force which is acting on the measurement lever 106. The position of the measurement lever 106 is measured by means of an optoelectronic measurement device 111 which is connected to a controller device 10′ that regulates the compensating current Icomp in response to the incoming measurement signals in such a way that the measurement lever 106 is always held at a constant position or is returned to the same position after a change of the load.
The permanent magnet system 109 is arranged in a console 104 which is connected through flexure pivots 103a by way of parallel-guiding members 103 to a hanger 101 which includes a cantilevered extension 101a serving to receive a load to be measured. The vertical component of the force generated by the load is transmitted from the hanger 101 through a coupling 105 to the measurement lever 106 which is suspended at a part 104b of the console 104 by means of a flexure fulcrum 107.
To measure the position of the measurement lever 106, the optoelectronic measurement device 111 includes two photodiodes D1, D2 which are arranged opposite a light-emitting diode D3 on the inside of an angular bracket 104a that is attached to the console 104. The space between the photodiodes D1, D2 and the light-emitting diode D3 is traversed by an end section 106b of the measurement lever 106 which is configured as a light barrier or slotted aperture vane and includes an aperture slot 106a, so that radiation emitted by the light-emitting diode D3 can pass, dependent on the position of the light barrier, through the aperture slot to the photodiodes D1, D2 which generate corresponding photo currents I1 and I2 as inputs to the controller device 10′. When the measurement lever 106 is deflected out of its initial position or normal position, either the first or the second photodiode D1, D2 receives more radiation, and as a result, the controller device 10′ receives photocurrents I1 and I2 of unequal magnitude.
The controller device 10′ includes a differential measurement circuit 1′ which, based on the two currents I1 and I2, generates a differential voltage uΔ which vanishes in the case where the photo currents I1 and I2 are equal. The differential voltage uΔ is directed to a driver circuit 2 that follows downstream in the circuit path and causes a corresponding compensating current Icomp to flow by way of a reference resistor 3 through a coil 110, whereby a counterforce is generated which corresponds to the load and returns the measurement lever 106 to the initial position. The voltage generated across the reference resistor 3 by the compensating Icomp is received by a converter module A/D and converted to a corresponding digital value which is presented on a display unit DSP.
Further examples of differential measurement circuits that can be used to produce a differential signal out of two signals coming from two photodiodes are disclosed for example in [2] U.S. Pat. No. 3,727,708 and [3] DE 2 311 676.
A contact-free displacement transducer producing a voltage signal which is described in reference [3] includes a differential photodiode consisting of two individual diodes, designed to receive optical radiation passing through the slot of a movable light barrier vane, for example the slot 106a in the end section 106b of the measurement lever 106. In this circuit arrangement, the photo currents generated by the photodiodes are directed, respectively, to a first and a second operational amplifier which produce voltages that are proportional to the photocurrents and are applied to the input terminals of a third operational amplifier which, in turn, delivers a differential signal at its output terminal which is proportional to the difference of the output voltages of the first two operational amplifiers and proportional to the displacement distance of the aperture slot.
To generate the differential signal, the differential circuits described in [2] and require several resistors and operational amplifiers whose temperature- and operating behavior can have a detrimental effect on the resultant differential signal, whereby measurement errors can be caused in a balance with electromagnetic force compensation. Operational amplifiers often have undesirable offset voltages which, like resistors, can be subject to disturbing fluctuations in the presence of an often unstable ambient temperature.
The differential measurement circuit 1′ illustrated in FIGS. 2a and 2b which is designed to receive the photo currents I1, I2 generated by the photodiodes D1, D2 has been proposed in [1] as a way to avoid the drawbacks of the circuit arrangements disclosed in [2] and [3]. The differential measurement circuit 1′ generates an output signal uΔ which is proportional to the difference of the photo currents I1, I2 and is largely independent of fluctuations of the ambient temperature. To accomplish this, a switch SW which is connected to the two photodiodes D1, D2 and the inverting input of a first operational amplifier OAΔ is periodically flipped by a controller unit CTRL between two states zt1, zt2. As a result, in the first state zt1 during a first phase t1 of the period, the inverting input of the first operational amplifier OAΔ is connected through a first node KΔ to the first photo current I1; and in the second state zt2 during a second phase t2 of equal length, the inverting input of the first operational amplifier OAΔ is connected through the first node KΔ to the second photo current I2. The output terminal of the first operational amplifier OAΔ at which the output signal uΔ is generated is connected back to the inverting input by means of a resistor RΔ running parallel to a capacitor CΔ so that the operational amplifier OAΔ works as a time-delayed proportional controller.
The inverting input of a second operational amplifier OAΣ which is connected to a second node KΣ receives a reference current I0 from a reference voltage source U0 by way of a reference resistor R0, so that at the output of the further operational amplifier OAΣ a summation voltage UΣ establishes itself dependent on the difference between the photo currents I1, I2 flowing during the first phase of the time period and the reference current I0 flowing during the first and second phase of the time period, wherein the summation voltage UΣ depends on the respective magnitudes of the reference current I0 and the photo currents I1, I2. The second operational amplifier OAΣ, whose output is connected by way of a capacitor CΣ to the inverting input, works as an integrator. By regulating the operating voltage of the light-emitting diode D3 dependent on the summation voltage uΣ the photo currents I1, I2 are kept constant.
In the circuit arrangement according to [1], which is of a relatively simple design, the photo currents I1, I2 are applied to the differential circuit 1′ sequentially. Consequently, the difference is established with a corresponding delay and the information of the photo current I1 or I2 that was not connected in the respective time phase is lost. A further delay is caused by the first operational amplifier OAΔ which operates as a time-delayed proportional controller. Furthermore, the voltage at the cathodes of the photodiodes D1, D2 is not defined and may change at the flip of the switch SW. The symmetry of the circuit, wherein the first photo current I1 flows through the virtual mass at the inverting input of the second operational amplifier OAΣ while the second photo current I2 flows through the actual mass at the anode of the second photodiode D2, is not assured.
Balances with electromagnetic force compensation of the kind described above have reached a high level of technical maturity. The applied technology is simple and efficient. Consequently, any progress in this technology can only be realized with a considerable effort.
The present invention therefore has the objective to provide an improved differential measurement circuit for a balance with electromagnetic force compensation as well as an improved balance with electromagnetic force compensation.
The aim is in particular to provide a differential measurement circuit which has on the one hand a simple design configuration and on the other hand delivers a precise output signal quickly and reliably.
The differential measurement circuit should maintain a stable operating behavior under all possible conditions independent of switching status. The individual components should be working with defined operating parameters that are not affected by switching processes. The goal of the invention is a simple symmetric circuit arrangement that is resistant to extraneous disturbances.
Where photodiodes are used, the circuit should be designed to allow them to constantly work within an ideal operating range. The switching of photo currents which could cause disturbances should preferably be avoided.
A loss of information in the evaluation of the measurement signals should be avoided.
Furthermore, the circuit should deliver an output signal that is advantageously suited for further processing.
The balance with electromagnetic force compensation according to the invention should be capable of being adapted to different fields of application, in particular with a view to improvements in counteracting disturbances occurring in the different fields of application and further improving the measurement results.